1. Field of Invention
This invention relates to a coaxial coupling apparatus for coupling two bodies together along an axis to prevent relative rotational and axial movement of the bodies. It may be particularly advantageous on a downhole bore assembly, and more particularly on a gap sub member thereof.
2. Description of Related Art
In many fields it is necessary to couple two bodies together axially such that the bodies remain axially aligned and are prevented from relative rotational and axial movement. This is particularly true in downhole bore equipment such as a downhole bore assembly and more particularly in a gap sub component thereof where it is necessary to electrically isolate first and second portions of a drill string so that such portions can be used as opposite poles of an antenna to transmit signals into the earth for reception by a receiver located farther up or down the drill string or for reception by surface-based signal detection equipment.
Conventional technologies for connecting together two portions of a gap sub that may be connected to respective opposite portions of a drill string may involve the use of opposing grooves in complementary portions of the gap sub that form a screw thread of constant pitch. The thread may be somewhat loosely filled with ceramic balls held in place by a thermoplastic material, for example. The ceramic balls mechanically couple the complementary portions together to prevent relative longitudinal movement of the complementary portions, but relative rotational movement is impeded only by the thermoplastic material. This places severe reliance on the integrity of the thermoplastic material to prevent rotational movement. Such reliance can be dangerous because the thermoplastic material is significantly weaker at elevated temperatures and can be somewhat susceptible to fatigue and degradation due to temperature cycling, corrosive fluids and mechanical fatigue when used in a downhole bore environment.